Are you talking about rejects or HW errors. Let's not make this the same thing.
You have two types of rejects; stale share and share below target. First one happens when you are too late to the party (basically, too high ping from you to the pool), second one happens due to incorrect configurations, wrong algorithm and similar issues. Which ones are you getting?
When it comes to HW errors, read my post above.
If you understand how stratum protocol works, how job delegation is performed, then you can clearly understand that ANY job throw at miner HAS to be performed with 0 HW errors. What is job at all? It is a string of data, that is being "hashed". Now you tell me, how can this string of data affect HW error rate? The only explanation is bad mining software that is perhaps incapable of doing fast work switch thus doing some nasty things it shouldn't (overwrites?) which results in HW errors thus rejected shares.
Unfortunately, we cannot control what kind of orders are put on NiceHash and some buyers put orders with fast work switching pools.
Shouldn't you be able to pick one address from your database with a high reject rate and find out what the pool is getting from the miner? Anyway, here it is:
[01:09:53] Stratum from Pool 0 requested work restart
[01:09:53] Pool 0 stale share detected, submitting as user requested
[01:09:53] Pool 0 stale share detected, submitting as user requested
[01:09:53] Rejected 038413a0 Diff 1.03K/128 GPU 0 Pool 0 (Job not found.)
[01:09:53] Rejected 024cb076 Diff 304/128 GPU 1 Pool 0 (Job not found.)
Round started UTC 2014-04-28 18:17:38
Round accepted GH 43.60
Round rejected GH 7.52 (14.70%)
Now I don't know how else I could possibly convey to you that Gridseed HWs are not necessarily hardware errors, but possibly rejects/stales by another name, so lets not bother with that. I'm not too fond of their software either so let's blame it
The above is from an R9 rig with zero HWs, so nothing to do with Gridseeds anyway. Just give me a reasonable explanation how a perfectly fine mining rig can have a 10-15% sustained reject rate on nicehash but not on any other pool, and - more importantly - way above what you have stated "plenty of miners" are having. If it's geography maybe it's time to consider multiple endpoints. If it's something else I would like to know and I think I'm not alone. I can poke in the dark here trying different rigs and even different locations but that's a waste of time.
